The present invention relates to a dielectric filter comprised of ceramic material, and more particularly to a dielectric filter and its method of manufacturer, to which radio frequency signals (hereinafter referred to as RF signals) having a frequency from the ultra high frequency (UHF) bands to the relatively low frequency microwave bands can be coupled, and which is well adapted for a bandpass filter coupling to RF signals having either of the frequency ranges from 825 MHz to 845 MHz or from 870 MHz to 890 MHz, which are used by mobile telephones.
A dielectric filter must be tuned after the filter is initially constructed and tested. A conventional dielectric filter structure whose frequency response may be finely adjusted is described in detail in U.S. Pat. No. 4,431,977 and Japanese laid-open Patent Publication No. 84-128801. A fine frequency adjustment of the filter described in U.S. Pat. No. 4,431,977 is performed by removing an amount of the conductive material from around the conductor-lined holes formed in the dielectric material, the amount of the material removed determining the amount of adjustment.
There has been a continuing effort, particularly in the field of mobile telephones, to reduce the size of the filters. A problem arises, however, in reducing the size of a filter which is tunable in the manner of the prior art because the amount of conductive material to be removed for a given adjustment will be necessarily decreased, and thus the removal process is more sensitive and therefore more time consuming and expensive.
Another adjustment approach which is described in Japanese laid-open Patent Publication No. 84-128801 is to perform the fine frequency response adjustment of the filter by cutting conductive strip lines which are provided on the top surface, surrounding the holes. This other adjustment approach may be used to finely adjust the frequency response of the filter. However, it has been found that with this approach, portions of the ceramic material provided between the holes and the strip lines reduce the unloaded Qu of the filter.